A control system capable of performing ignition retard control is known which may be applied to an internal combustion engine installed on a vehicle equipped with a brake booster operable to increase the force applied to a brake system (which will be called “brake operating force”) so as to increase the braking force of the vehicle, by utilizing a difference between the pressure (downstream-of-throttle pressure) of air in an intake passage downstream of a throttle valve and the atmospheric pressure. During startup of the engine, the control system performs control (ignition retard control) for retarding the time (ignition timing) at which a spark is produced in a combustion chamber relative to the normal ignition timing, so as to promote warm-up of an exhaust treatment catalyst disposed in an exhaust passage and thereby bring the catalyst into an activated state.
During the period in which the ignition retard control is performed, torque (output torque) produced by the engine supplied with the same amount of a fuel-air mixture is reduced due to deterioration of the thermal efficiency, as compared with the case where the ignition retard control is not performed. Therefore, the control system increases the degree of opening of the throttle valve (throttle opening) to a value larger than the normal throttle opening during this period. With the throttle opening thus increased, the amount of air introduced into each combustion chamber (cylinder) (which may also be called “in-cylinder air amount”) and the amount of fuel supplied in proportion to the air amount are increased, so that the output torque produced during the period of the ignition retard control can be made substantially, equal to output torque produced when the ignition retard control is not performed.
In the meantime, as the throttle opening increases, the pressure downstream of the throttle valve gets closer to the atmospheric pressure. In the control system as described above, therefore, the brake booster may not be able to sufficiently increase the brake operating force due to a reduced difference between the pressure downstream of the throttle valve and the atmospheric pressure during the period in which the ignition retard control is performed, which may result in a shortage of the braking force of the vehicle. In view of this situation, a control system disclosed in Japanese Patent, Application Publication No. 2002-327639 (JP-A-2002-327639) is adapted to stop execution of ignition retard control by controlling the throttle opening to the normal throttle opening and controlling the ignition timing to the normal ignition timing when a relatively large braking force is required, even during the period in which the ignition retard control is to be performed so as to promote warm-up of the catalyst. In this manner, a shortage of the braking force of the vehicle is avoided.
Immediately after the execution of the ignition retard control is stopped and the throttle opening is reduced to the normal throttle opening, air having a relatively high pressure remains in a portion of the intake passage downstream of the throttle valve. Therefore, the actual in-cylinder air amount (the actual amount of air drawn into the cylinder) becomes equal to the amount corresponding to the reduced throttle opening with a delay, namely, after a time lag from the time of change of the throttle opening. Accordingly, immediately after the control system starts inhibiting the execution of the ignition retard control, the in-cylinder air amount may become excessively large, and the output torque may become excessively large, resulting in the occurrence of torque shock.